This invention relates to a cryopump in which the molecules of gas, having a boiling point higher than the temperature of a cryopanel cooled to extremely low temperatures, are condensed and adsorbed on the surface of the cryopanel, thereby exhausting a large amount of the gas at high speed.
In a conventional apparatus as disclosed in Japanese Patent Unexamined Publication No. 61-169682, a plurality of cryopanels cooled to extremely low temperatures by liquid helium or the like are disposed perpendicularly to a plane of a gas inlet and are spaced a predetermined distance from one another, with each of the cryopanels having a single flat surface. Front shield plates cooled by liquid nitrogen are provided at a part of the gas inlet so that the cryopanels will not be heated by the radiation heat applied from a high-temperature portion in the vicinity of the gas inlet. Louver blinds cooled by liquid nitrogen are provided at the sides of the cryopanels. The gas molecules pass through the gaps in the louver blinds and are condensed and adsorbed on the cryopanels. A rear shield plate cooled by liquid nitrogen is also provided at the rear side of the cryopanels.
The surfaces of the front shield plates, the rear shield plate and the louver blinds are treated to have a black color so that these surfaces can adsorb the radiation heat. External rays of light are once reflected by these shield plates, and then reach the cryopanels.
The gas molecules to be exhausted flow into the cryopump via inlets each formed between two adjacent front shield plates. The gas molecules impinge on the louver blinds or the rear shield plate once or several times, and some of the gas molecules flow out of the inlet whereas other gas molecules reach the cryopanels and are condensed and adsorbed thereon. If the rate or proportion of such condensed and adsorbed gas molecules is increased, the exhaust speed of the cryopump can be increased.
In the conventional cryopump, the louver blind surface (surface A) is disposed at a certain angle .alpha. with respect to the plane of a gas inlet and a line normal to the louver blind surface is directed toward the gas inlet. It is most probable that the reflected gas molecules advance in the direction normal to the impinging surface according to the cosine law. Therefore, after the gas molecules impinge on and are reflected by the surface A of the louver blinds, the number of those gas molecules which move back toward the gas inlet is larger than the number of those gas molecules which are directed toward the cryopanel. Particularly, those gas molecules, impinging on the surface A of the louver blinds disposed near the gas inlet, flow back through the gas inlet at a higher rate after they are reflected by the surface A. Thus, the conventional cryopump suffers from the problem that the rate or proportion of the gas molecules condensed and absorbed on the cryopanels, namely, the molecule arresting rate is small, which results in failure to increase the exhaust speed.
Further, in the conventional louver blind-type cryopump, since the cryopanel has a single flat panel surface, gases of a low-boiling point, such as helium gas, which can not be condensed and adsorbed on such a single panel surface, can not be exhausted.